1. Field of the Invention
The present invention relates to a video camera with a circuit for correcting flare of a video signal.
2. Description of the Prior Art
In photographing with a video camera, there may occasionally occur a phenomenon termed flare where incident light from a subject being photographed is partially reflected with diffusion inside the camera and so forth to consequently cause whitish fog of the subject image. Such flare is rendered more conspicuous in accordance with increase of high luminance portions in the subject and deteriorates the quality of the reproduced image.
For the purpose of preventing such undesired phenomenon, a video camera is generally equipped with a flare correcting circuit so as to correct any flare of the video signal.
FIG. 1 shows an exemplary constitution of a video camera including from an image sensor to a flare correcting circuit. In this diagram, an output signal S1 of a CCD image sensor 1 is supplied to a sampling-holding circuit 2, from which an effective video signal S2 is delivered to a pre-knee circuit 4 via a gain control circuit 3.
The pre-knee circuit 4 has such input-output characteristic as shown in FIG. 2 so that, when the level of the input video signal S2 has exceeded the pre-knee point, the gain relative to the output video signal S4 is reduced. Therefore in the video signal S4, the level of its high luminance component is compressed more than that of the video signal S2.
The video signal S4 is supplied to an analog-to-digital (A-D) converter 5, where the signal S4 is converted to a digital video signal S5 of which one sample is composed of parallel 10 bits for example, and then the video signal S5 is supplied to a flare correcting circuit 6.
This flare correcting circuit 6 consists of a subtracter 61 and an integrator 69 with a time constant ranging from a period of several fields to one second or so, wherein the video signal S5 from the converter 5 is supplied as a main signal to the subtracter 61 while being supplied also to the integrator 69, from which a DC component S69 (digital signal corresponding to the DC component of the signal S4) is outputted and then is supplied to the subtracter 61.
In this case, the DC component S69 is proportional to the whitish fog of the subject image derived from the flare. And in the subtracter 61, the DC component S69 is subtracted from the video signal S5, so that the resultant signal S6 is the one obtained by removing from the original video signal S5 the whitish fog caused by the flare. Namely, the signal S6 is a flare-corrected video signal.
The video signal S6 thus obtained is supplied to a gamma correcting circuit 7 for correction with respect to the gamma, and the corrected signal is outputted from a terminal 8.
However, in the flare correcting circuit 6 mentioned above, there may occur insufficiency in the amount of flare correction if any high luminance portion is existent in the subject being photographed.
When the subject is a high-luminance, white disk on a black ground illustrated as "a" in FIG. 3 for example, a video signal S4 representing the subject has a waveform as shown at "b" in FIG. 3 with changes of its level. And if the allowable input level (dynamic range) of the A-D converter 5 is V5 in comparison with such video signal S4, it follows that the signal S4 is clipped at the level V5 and merely the shaded portion in FIG. 3 is converted into a digital video signal S5.
Therefore, when a flare correction signal S69 is formed from such video signal S5, the non-shaded portion above the level V5 of the signal S4 is not used for forming the flare correction signal S69, whereby insufficient correction is induced due to employment of such flare correction signal S69.
In this case, although a pre-knee circuit 4 is provided in the preceding stage of the A-D converter 5, the level of some high luminance component of the video signal S4 may exceed the allowable input level V5 of the converter 5 since the pre-knee characteristic is not rendered flat in the high luminance component.
The above problem is eliminable if the peak value of the signal S4 supplied to the A-D converter 5 is reduced to be lower than the allowable input level V5 by adjusting the gain of the gain control circuit 3.
However, the resolution at the conversion performed by the A-D converter 5, i.e. the range of the analog level per bit, is widened to consequently cause an increase in the quantization error. Namely, due to the high luminance component of the video signal, the quantization at the visually conspicuous gray level is rendered rough.